In its most general aspect the present invention relates to the production of urea.
Specifically, the present invention relates to a process for decomposing a carbamate aqueous solution coming from the urea recovery section of a urea production plant. The above carbamate aqueous solution is also hereinafter referred to as xe2x80x9crecycle carbamate solutionxe2x80x9d.
In the art, a series of urea production processes are known essentially based on the performance of a conversion reaction in a reaction space or reactor fed with ammonia and carbon dioxide, and of a recover of the unreacted substances contained in the urea solution leaving the reaction space, in particular ammonia, carbon dioxide and carbamate in aqueous solution, which are recycled to the reaction space.
To this purpose, a plant implementing the above process comprises downstream to a reaction space, a carbamate decomposition unit, operating at substantially the same pressure as the reaction space, for subjecting the urea solution leaving the reaction space to a treatment of partial decomposition of the carbamate and partial separation of free ammonia. It is thus obtained a flow comprising ammonia and carbon dioxide in vapor phase, which is recycled to the reaction space and a flow comprising urea and residual carbamate in aqueous solution.
The plant also comprises a urea recovery section for separating the residual carbamate from the urea, obtaining carbamate in aqueous solution, which is recycled to the reaction space.
The carbamate decomposition unit is generally an apparatus comprising a vertical tube bundle. The carbamate decomposition is performed by heating the urea solution flowing downwards as a falling film inside the tubes and in contact with their internal walls, with a heating fluid such as steam flowing outside the tubes. In this way, unreacted ammonia and carbon dioxide are stripped from the urea solution (thermal stripping).
In case of carbon dioxide stripping, the carbamate decomposition is also carried out by means of a gaseous flow comprising carbon dioxide feed flowing upwards inside the tubes.
An improved process for the production of urea, which is disclosed in EP-A-0 796 244, comprises the addition of a partial decomposition step of the carbamate aqueous solution obtained in the urea recovery section and recycled to the reaction space.
In this way, it is possible to remarkably reduce the amount of water recycled to the reaction space, thus permitting to obtain an increase in the conversion yield and in the production capacity of the plant implementing this process.
To this purpose, the plant is provided with a decomposition unit for subjecting at least part of the carbamate solution coming from the urea recovery section (recycle carbamate solution) to a treatment of partial decomposition or stripping, obtaining an additional flow comprising ammonia and carbon dioxide in vapor phase, which is condensed and recycled to the reaction space.
Although advantageous in many aspects, a drawback which is encountered when operating the above decomposition unit is that the decomposition efficiency is unsatisfactory.
According to the studies carry out by the applicant it has been found, not without surprise, that the film of recycled carbamate solution flowing downwards into the tubes bundle is subjected to undesired breakage and detachment from the tube internal walls, in particular at an upper portion thereof. Where this happens, no wetting of the internal wall is achieved.
This causes a low effective thermal exchange between the heating steam and the liquid film of recycled carbamate solution flowing inside the tubes. The stripping efficiency of the decomposition unit of the recycled carbamate solution is thus lower than expected.
Because of that, in order to have acceptable decomposition efficiency, the decomposition unit need to be manufactured with greater dimensions than those theoretically required, with ensuing high investment costs.
The technical problem underlying the present invention is that of improving the decomposition efficiency of a decomposition unit of a urea production plant delegated to the stripping (decomposition) of a recycle carbamate solution.
To solve the above problem, the basic idea underlying the present invention is that of modifying the prior art processes for decomposing the recycle carbamate solution in such a way to improve the efficiency of the decomposition units delegated to this decomposition, rather than modifying the structure of these units, which would be complex and involve considerable costs.
Based on the above idea, the problem underlying the present invention is solved by a process for decomposing a carbamate aqueous solution coming from the urea recovery section (recycle carbamate solution) according to the annexed claims 1 to 5.
The recycle carbamate solution generally has a water concentration within the ranges of 10% and 70% and has a temperature of from 70xc2x0 C. to 120xc2x0 C. at the outlet of the urea recovery section.
Thanks to the present invention a substantial increase of the decomposition efficiency can be obtained since it is possible to control the evaporation of the film of recycle carbamate solution flowing in the tube bundle of the decomposition unit.
In particular, it has been surprisingly found out that the detachment of the film of recycle carbamate solution from the tube walls of prior art units is principally due to the fact that this solution is subjected to an excessive and violent evaporation in the tubes, especially in the upper portion thereof. In other words, it is such unexpected evaporation that disturbs the normal downward flow of the liquid film in the tubes.
In order to effectively hinder these evaporation phenomena, it has been proposed according to the invention to advantageously reduce the high temperature difference between the operating temperature of the heating fluid (e.g. 210xc2x0 C.) and the temperature of the recycle carbamate solution entering the tubes of the decomposition unit (70-120xc2x0 C.).
According to the invention, it has been surprisingly found out that if the above temperature difference is not higher than 70xc2x0 C., preferably between 20xc2x0 and 40xc2x0, the undesired violent evaporation phenomena are substantially eliminated and, at the same time, an increased decomposition efficiency of the decomposition unit is obtained.
This result is totally unexpected if one considers that, in principle, a low temperature difference between the temperature of the recycle carbamate solution and the operating temperature of the heating fluid is expected to reduce the carbamate decomposition into gaseous ammonia and carbon dioxide.
Further characteristics and advantages of the present invention are set forth in the detailed description of a preferred embodiment thereof given below by way of non-limiting example with reference to the annexed drawings.